Interpretive Summary: In alfalfa breeding, selection efficiency is rarely examined. Therefore, it is largely unknown if current widely practiced selection methods are most efficient in terms of resources and time usage. This study indicates that selection for plant size, yield, and regrowth ability should be most effective during the latter part of the growing season. This study also indicates that selection during the latter part of the growing season should improve alfalfa cultivar performance during the whole growing season. Implementing these results should lead to more rapid improvement of alfalfa cultivars.

Technical Abstract:
The complex genetics of autotetraploid alfalfa make additive variance component estimation difficult. Most often between halfsib family variances are used to estimate additive genetic variances and by extension narrow sense heritabilities and additive genetic correlations. These estimates contain a portion of the dominance variance. In such calculations, in conjunction with parent-offspring covariance estimates, the dominance component can be separated from the additive genetic component. This is rarely done. This study reports average estimates across thirty populations, of both additive and dominance variance component estimates based on between halfsib family variance and parent-offspring covariance for biomass yield, plant height, regrowth, plant width, plant growth angle, vegetative density, and maturity during each of three harvests. Almost consistently we found negative dominance variance estimates. Based on previous theory this suggests epistatic interactions are a noticeable component of most traits measured. Assuming no epistasis, a routine assumption in autotetraploid alfalfa leads to inflated narrow sense heritability estimates when compared to estimates based on parent-offspring regression. Assuming no epistasis and no dominance variance, weighted averages of additive between halfsib family and parent-offspring effects revealed plant width and vegetative density additively correlated with biomass yield. Peak photoperiod maturity had a slight negative additive correlation with biomass yield. Plant height had no additive correlation with biomass, in contrast to the strong phenotypic correlation observed. Additive genetic correlations for the same traits measured during different harvests in most cases were highly correlated. On average third harvest heritabilities were greatest. These two results suggest selecting plants based on later season performance (August - October) is most effective for Iowa environments.